Prolonged stress, acting through the exaggerated hypothalamic-pituitary-adrenal (HPA) axis with high levels of glucocorticoids, may be causally related to the onset of depression. Dysfunction of noradrenergic neurotransmission is also thought to be involved in the development of depression. Restoring deficient norepinephrine (NE) in noradrenergic synaptic clefts has been considered to contribute to therapeutic effects of antidepressants with specific inhibition of the NE transporter (NET). However, the molecular correlation between stress and noradrenergic neurotransmission, two potential etiological factors for depression, is poorly understood. It is possible that stress may dysregulate the noradrenergic system thereby contributing to the pathophysiology of depression. Our recent work demonstrates that chronic stress significantly increases NET expression and function in the rat LC and its terminal regions (hippocampus and amygdala). Similar results are also found in rats and cultured cells, which were treated with or exposed to stress-relevant doses of corticosterone. These findings suggest that stress-induced alterations of NET may serve as the point of entry for stress-triggered precipitation of depression. Therefore, we hypothesize that stress, possibly acting through corticosterone, stimulates the expression and function of NET in central noradrenergic neurons by transactivation of the NET gene. Such an effect could result in a functional deficiency of NE in the synaptic clefts thereby contributing to altered noradrenergic transmission in depression. In this proposal, in vivo and in vitro studies will be conducted to accomplish four specific aims: (1) To use a rat stress model of chronic social defeat to assess stimulatory effects of stress on the NET expression and function in the LC and its key terminal regions;(2) To use normal rats treated with stress relevant doses of corticosterone to clarify stimulatory effects of corticosterone on the NET gene;(3) To use cultured cells to validate the in vivo findings;(4) To determine the molecular mechanisms underlying the regulation of NET by corticosterone. In all steps, possible reversed effects of the antagonists of corticosteroid receptors on corticosterone-induced NET regulation will be evaluated. The proposed studies will elucidate the molecular link between stress, corticosterone, and the NET gene, as well as the transcriptional mechanisms. The findings will augment our understanding of causal roles of stress and noradrenergic dysfunction in depression, and ultimately may lead to novel pharmacotherapies for this disease. Chronic stress is one possible cause of major depression. During stress, there is an increase in release of hormones such as corticosterone. These hormones affect many brain areas functionally and structurally. Norepinephrine transporter is a key protein in the brain and is related to the function of the noradrenergic system which controls emotion, memory and cognition. The goal of this project is to elucidate the regulatory effects of stress, stress hormones on the norepinephrine transporter, as well as underlying molecular mechanisms. A rat stress model of chronic social defeat and the cell models treated with corticosterone will be used. The results will improve our understanding of causal roles of stress and noradrenergic dysfunction in depression, and ultimately may lead to novel pharmacotherapies for this disorder.